A gamma ray is a high-energy electromagnetic emission by certain radionuclides when the state of those certain radionuclei transitions from a higher to a lower energy state. Gamma rays have high energy and a short wave length, with energies above 1 million eV and wavelengths less than 0.001 nanometers. All gamma rays emitted from a given isotope have the same energy, which has historically enabled scientists to identify which gamma emitters are present in an unknown sample.
Gamma rays, as well as protons, alpha particles, beta particles and x-rays, may cause direct ionization in that these particles or rays transfer at least a portion of the energy thereof upon interaction with matter. This transfer generally occurs by imparting energy to electrons of atoms that have been interacted with. Generally speaking, these ions may be measured by using measuring devices, such as a Geiger counter, for example.
While beta and alpha particles each have mass and charge, and are natural products of the decay of, for example, uranium, radium, polonium, and many other elements, gamma and x-rays have no mass and no electrical charge. Each is thus pure electromagnetic energy.
Most gamma and x-rays can easily travel several meters through the air and penetrate several centimeters of human tissue. Some emissions have enough energy to pass through the body, exposing all the organs to radiation. Gamma emitting radionuclides do not have to enter the body to be a hazard, as direct external and internal exposure to gamma rays or X-rays are of concern.
A large portion of received gamma radiation largely passes through the body without interacting with tissue, as the body is mostly empty space at the atomic level, and gamma rays are atomically small in size. By contrast, alpha and beta particles inside the body lose all their energy by colliding with tissue and causing damage. X-rays may act in a manner similar to alpha and beta particles, but with slightly lower energy.
Gamma rays do not directly ionize atoms in tissue. Instead, they transfer energy to atomic particles such as electrons (which are essentially the same as beta particles). These energized particles then interact with human tissue to form ions, in the same way radionuclide-emitted alpha and beta particles would. However, because gamma rays have more penetrating energy than alpha and beta particles, the indirect ionizations they cause generally occur further away from the emission source, and consequently, deeper into human tissue. Sources of gamma rays typically include radioactive elements such as Thulium 170, Iridium 192, Cesium 137, and Cobalt 60, while sources of x-rays typically include x-ray tubes within the controlled environment of a medical office.
While there are many beneficial uses for radioactive materials in the fields of science and medicine, these materials may be highly threatening to society. It goes without saying, radiation poisoning may be a tactic of terrorist groups and other radical factions with the intent to bring harm or even death to others. For example, the use of “dirty bombs”, which add radioactive materials to common explosives, has been well documented. Other possibilities, such as the contamination of food stocks or water sources with radioactive materials, have also been speculated.
The U.S. government does not take these sorts of potential threats lightly. For example, risk priority matrices set forth by the U.S. government include Cs 137 and Co 60, because of the large quantities of these isotopes that exist and, in the case of Cs 137, the ease of dispersal. Sr 90, Pu 238, Am 241 and Ir 192 are also included in the matrix of potential threats. In addition, spent fuel is generally included in potential threat matrices, and needless to say there are very significant quantities of spent fuel available.
Because nuclear devices or threats such as those described above may be assembled or deployed at any location, it would be advantageous for authorities to have the capability of sensing radionuclides at widely dispersed locations. By way of nonlimiting examples, such locations may include automotive highways, bridges, airports, train stations, municipal mass transit systems, governmental buildings, freight handling facilities, and the like. Automating the screening or sensing at such sites may enable the screening at those sites to be free of human intervention when no radionuclides are detected, and yet may readily enable the alerting of appropriate authorities upon a positive detection and/or identification of a specific radionuclide deemed to be a threat.
To date, there are several types of detectors, each having varying degrees of resolution and performance. For example, the differences in performance characteristics of sodium iodide (NaI) versus Germanium for gamma ray spectroscopy have been well characterized. However, the increased resolution of germanium detectors, obvious upon visual inspection of the spectra, can be illusive when evaluating the advantages for systems that might automatically identify radionuclides within spectra. Many gamma spectroscopy based sensors have and will be deployed as standalone, automated surveillance/detection systems, a reality that places the performance and reliability of automatic radionuclide identification systems at central and increasing importance.
Traditional automated, peak-fitting algorithms for identifying radionuclides in gamma-ray spectra may work in a very similar manner to that of the human eye in determining specific radionuclides. When employing these conventional tools, nuclear spectroscopy data derived from scintillators may often prove to be indeterminate as to the identification of originating specie. The problem of identifying embedded spectra, while difficult for the unaided eye and corollary conventional algorithms, is subject to acceptable resolution when it is addressed with more sophisticated algorithm based systems.
Thus, there remains a need for automated systems and methods to detect and identify any of a wide range of radionuclides from complex or “noisy” spectral data in a cost-effective manner.